Closing In on the Cosmos's Biggest Blasts

When a massive earthquake strikes, no sane seismologist would try to trace the rumble of a passing truck amid the noise of the aftershocks. But astronomers may have pulled off an equally challenging feat: detecting the glimmer of a supernova explosion in the fading afterglow of a titanic gamma ray burst (GRB)--one of the biggest type of explosions in all the cosmos. The observations by the two teams, however, each back a different theory of how supernovas might be related to GRBs.

According to the popular "collapsar" theory, a GRB occurs when a very massive star explodes as a supernova and collapses into a black hole. The theory predicts that as the burst fades, the debris from the supernova should temporarily brighten. Now, a team led by Joshua Bloom of the California Institute of Technology in Pasadena has found this faint supernova signature in recent Hubble Space Telescope observations of GRB 011121. The team describes its findings in an online paper submitted to The Astrophysical Journal Letters. According to theorist Martin Rees of Cambridge University in the United Kingdom, "it is definitely strong evidence" for the validity of the collapsar model.

But in a paper in the 4 April issue of Nature, a team led by James Reeves of the University of Leicester, U.K., reports findings that challenge the collapsar theory's assertion that the burst and supernova occur simultaneously. Reeves's team measured the spectral signature of a hot, expanding shell of supernova debris in the x-ray afterglow of another GRB, GRB 011211. Apparently, a massive star first exploded as a supernova, and then a few days later the ejected shell was heated by the energetic radiation of a subsequent GRB.

This sequence--first the supernova, then the GRB--is predicted by a theory of GRBs called the "supranova" model, which a minority of theorists prefer. In this model, the core of the dying star first collapses into a dense neutron star, triggering a supernova. The GRB occurs days, weeks, or even months later, when the neutron star further collapses into a black hole.

Bloom's findings may also be consistent with the supranova theory, according to Mario Vietri of the Third University of Rome in Italy, one of the theory's original authors. He says the brightening of the burst afterglow might be due to radiation from the GRB heating surrounding dust or reflecting off it. Stan Woosley of University of California, Santa Cruz, the father of the collapsar theory, agrees with Vietri that more observations are needed: "[Bloom's finding] smells like gunpowder, but it is no smoking gun [yet]."